PHASE TRANSITION MECHANISMS OF FERROELECTRIC ELPASOLITES AND LEAD ORDERING

G. Baldinozzi¹, A. Bulou², Ph. Sciau^1,3 and G. Calvarin¹

¹Laboratoire de Chimie Physique du Solide, URA CNRS 453, Ecole Centrale Paris, F 92995 Chatenay-Malabry Cedex, France. E-Mail: baldinozzi@cps.ecp.fr
²Laboratoire de Physique de l'Etat Condense, UPRES-A CNRS 6087, Universite du Maine, F 72085 Le Mans Cedex 9, France.
³Laboratoire CEMES, CNRS BP 4347, F 31055 Toulouse Cedex 4, France

Keywords: commensurate-incommensurate transitions, Raman spectroscopy, ferroelectricity and antiferroelectricity, disordered structures.

Complex lead-based perovskite ferroelectrics of general formula Pb₂B'B''O₆ presenting B'B'' ordering along <111> directions are called elpasolites. The space group of the high temperature prototype structures is cubic Fm3^_m and the simple perovskite cell edges are doubled.

Two kinds of disorder can be distinguished in complex perovskites: the former is related to the distribution of the two types of B cations in the B-type sublattices, the second is related to the existence of off centre displacements from the ideal positions in the prototype structure. It is well accepted that the former kind of disorder is responsible for the diffuse character of the phase transition in relaxor ferroelectrics. Here we want to point out that in compounds presenting cationic ordering (elpasolites), the second kind of disorder is connected to the order of the phase transition. In particular, when the shape of the multi-well potential seen by Pb atoms is changed at the phase transition, this is a first order one. On the other hand, when the topology of the disorder remains qualitatively unchanged, the phase transition is of the second order.

These considerations are illustrated by the analysis of the phase transition sequences of Pb₂CoWO₆ [1,2,3], Pb₂MgWO₆ [4,5] and Pb₂MgTeO₆ [6,7] by Raman spectroscopy, calorimetry, X-ray and neutron diffraction.

The different phase transitions (commensurate-commensurate, commensurate--incommensurate, incommensurate--incommensurate and incommensurate--commensurate) are described and interpreted in terms of condensation of soft modes. The deviations from the displacive character of these phase transitions are also pointed out. These results are also compared with other simple perovskites (BaTiO₃, SrTiO₃, LaAlO₃).

It is worth of notice that the low temperature orthorhombic phases of these compounds present a complete Pb ordering while the occurrence of a monoclinic or of a trigonal phase is associated to the onset of an incommensurate phase in which Pb atoms remain disordered suggesting the existence of a frustration when the orthorhombic structure does not take place. This feature is also observed in other complex perovskites presenting partly disordered B sublattices like PbSc_1/2Nb_1/2O₃ where Pb atoms remain disordered in the trigonal phase.

Therefore, ferroelectric materials can be located at the boundary among the physics of structural phase transitions, of incommensurate phases and of glass-like structures where the principal requirement of classical crystallography, namely the strict translational periodicity of the space structure, is broken by the presence of disorder.

1. Sciau Ph, Calvarin G, Sun B N and Schmid H, Phys. Status Solidi A 129 309-321 (1992).
2. Bonin M, Paciorek W, Schenk K J and Chapuis G, Acta Crystallogr. B 51 48-54 (1998).
3. Baldinozzi G, Sciau Ph, Calvarin G, Grebille D and Suard E, Rietveld refinement of the incommensurate phase of the elpasolite (ordered perovskite) Pb₂CoWO₆. (to be published).
4. Baldinozzi G, Sciau Ph, Pinot M and Grebille D, Acta Crystallogr. B 51 668-673 (1995).
5. Baldinozzi G, Sciau Ph and Bulou A, J. Phys.: Condens. Matter 7 8109-8117 (1995).
6. Baldinozzi G, Sciau Ph and Bulou A, J. Phys.: Condens. Matter 9 10531-10544 (1997).
7. Baldinozzi G, Grebille D, Sciau Ph, Kiat J-M, Moret J and Berar J-F, J. Phys.: Condens. Matter (1998) (at press).